QA/Networking

Testing IPv6 support and dualstack networking can be very tricky. The purpose of this document is to provide a live resource on testing situations arising from existance and usage of two network layer protocols.

Testing the base system

Base system components related to IPv6 and dual-stack operation:

• kernel
• glibc

Name resolution

Name resolution features are provided by the GNU C Library (glibc) which is not yet ready for proper IPv6 and dual-stack operation as you can see when performing your tests. The C library comes with its own testing tool getent that has a special database called ahosts that runs getaddrinfo(), the library function that translates names to objects with addressing information. For your testing it is best used together with tools like strace, ltrace or even gdb so that you know exactly what is happening behind the scenes.

$ getent ahosts www.fedoraproject.org raw
2604:1580:fe00:0:5054:ff:feae:702c STREAM wildcard.fedoraproject.org
2604:1580:fe00:0:5054:ff:feae:702c DGRAM  
2604:1580:fe00:0:5054:ff:feae:702c RAW    
2607:f188::dead:beef:cafe:fed1 STREAM 
2607:f188::dead:beef:cafe:fed1 DGRAM  
2607:f188::dead:beef:cafe:fed1 RAW    
2001:4178:2:1269::fed2 STREAM 
2001:4178:2:1269::fed2 DGRAM  
2001:4178:2:1269::fed2 RAW    
2610:28:3090:3001:dead:beef:cafe:fed3 STREAM 
2610:28:3090:3001:dead:beef:cafe:fed3 DGRAM  
2610:28:3090:3001:dead:beef:cafe:fed3 RAW    
66.35.62.162    STREAM 
66.35.62.162    DGRAM  
66.35.62.162    RAW    
140.211.169.196 STREAM 
140.211.169.196 DGRAM  
140.211.169.196 RAW    
209.132.181.15  STREAM 
209.132.181.15  DGRAM  
209.132.181.15  RAW    
152.19.134.142  STREAM 
152.19.134.142  DGRAM  
152.19.134.142  RAW    
67.219.144.68   STREAM 
67.219.144.68   DGRAM  
67.219.144.68   RAW    
67.203.2.67     STREAM 
67.203.2.67     DGRAM  
67.203.2.67     RAW    
209.132.181.16  STREAM 
209.132.181.16  DGRAM  
209.132.181.16  RAW    

Even in the simple example above you can see that the name resolution topic won't be as simple as one would guess.

When application requests addressing information for a hostname with an optional service name, the library returns a list of addressing information objects. The order of objects in the list is significant and depends on operating system configuration and connectivity.

Input from the application

• nodename
• servname
• protocol
• socktype
• flags
  • AI_CANONNAME
  • ...

Input from the local configuration

• To what extend is IPv4 and IPv6 available

Input from the outside world

• DNS information
• Multicast DNS information
• LDAP information

What is requested

Not all information is requested at all times. Some information like canonical name must be explicitly requested by the application via AI_CANONNAME flag. It may be desirable to suppress other requests by local configuration or connectivity checks, a notable example being suppression of DNS AAAA queries on hosts without global connectivity.

What is passed to the application

Not all information that is learnt via requests is presented to the application. It is typically filtered according to input from the application. It is sometimes also filtered according to connectivity checks but that has caused more problems than improvements.